Anti-icing of windows by dielectric heating



1956 o. M. CHRISTENSEN 2,773,162

ANTI-ICING OF WINDOWS BY DIELECTRIC HEATING Filed Jan. 14, 1954 HIGH FREQUENCY E L ecm ucm. ENERGY SOURCE 5, eo

INVEN TOR. Gem/v0 M. CWE/STE/VSEA/ V6 QQ /6a. 3

HIGH FREQUENCY E L ECTRICAL EN ERGY SOURCE United States Patent ANTI-ICING OF WINDOWS BY DIELECTRIC HEATING Orland M. Christensen, Edmonds, Wash., assignor to Boeing Airplane Company, Seattle, Wash., a corporation of Delaware Application January 14, 1954, Serial No. 404,045

7 Claims. (Cl. 219-10.81)

This invention relates to means for preventing formation of ice on the windows of airplane cabins and other housings. The term window as herein used has a broad connotation, including a plate or sheet of dielectric material which is transparent to electromagnetic or other radiant energy. The invention is herein illustratively described by reference to its presently preferred form as applied to the anti-icing of airplane windows, for instance; however, it will be understood that certain changes and modifications therein may be made without departing from the essentials involved.

It was proposed in the copending application of Roy V. Ostling, Serial No. 399,434, filed December 21, 1953, to utilize the dielectric heating effect of a high-frequency electric field passed through the body of a window panel to raise the temperature of the Window sufiiciently to prevent formation of ice thereon. The disclosed means for carrying out the method comprised two transparent electrically-conductive coatings deposited on respectively opposite sides of a window panel and serving as electrodes connected to opposite terminals of a high-frequency energy source. Window panels with coatings having the properties of both transparency and electrical conductivity suitable for use in the method are represented in the product of the Pittsburgh Plate Glass Company, referred to in the trade as Nesa, and in that of the Libby- Owens-Ford Glass Company, referred to as Electrapane. Also, there is disclosed in the patent to McMaster, No. 2,429,420, dated October 21, 1947, a method of pro ducing transparent electrically conductive coatings on glass panels.

Such a proposal is efficient for the intended purpose and offers certain distinct advantages over previous antiicing techniques. However, in terms of the apparatus disclosed in the patent application cited, it is observed that the placement of conductive film electrodes on both sides of the window panel called for additional overlay panels in order to cover and protect, as well as electrically insulate, both of the otherwise exposed conductive coatings. Further, the overlay panels themselves presented the surfaces exposed to the atmosphere, which surfaces were therefore required to be heated for antiicing purposes by a process of heat flow or conduction outwardly from the intermediate panel rather than directly by dielectric energy loss incurred in and along the composite panel exterior surfaces. Hence, the capability of the overlay panels to conduct heat from the intermediate panel wherein such heat was generated to the outer surfaces, imposed a limitation on the efiicient utilization of generated heat.

An object of the present invention is anti-icing apparatus utilizing the dielectric heating effect without the limitations just mentioned.

A more specific object is window anti-icing apparatus producing heat by dielectric energy loss incurred directly at and along the exposed window surface, where such heat is needed. In the case of an airplane window, for

2,773,162 Patented Dec. 4, 1956 ice instance, the present apparatus accomplishes the fore} going result at the exterior side of the window for anti icing purposes, and also at the interior side of the window for defrosting or anti-moisture condensation pur-' poses.

In accordance with the invention, a plurality of elongated mutually spaced electrodes are embedded in the window and arranged as a grid extending generally parallel to the window surfaces. Alternate electrodes are connected together and to the respective terminals of a high-frequency energy source. An alternating electric field is thereby established in the body of the window which fringes or strays outwardly from the general plane of the grid on both sides thereof and generates heat throughout substantially the entire thickness of the window, including the surface regions thereof.

The electrode grid may be formed of narrow strips of the above described electrically conductive transparent film material, or of very fine wires. In the former case visibility through the window is not impaired at all by the electrode strips while in the second case, employing wires, the obstruction to visibility for most purposes will be negligible. In the first case, in which the electrode strips are of electrically conductive transparent film material, the window is most practicably manufactured from two panels joined together, with the face of one panel carrying the coating strips and with the other panel adhesively bonded thereto as a protective overlay. In the case of fine wire electrodes, the window is most practicably manufactured by molding the grid of wires directly within a single panel or thickness of glass or other dielectric window material. In either case, both surfaces of the window will be heated dielectrically, and by spacing the electrode grid by different relative distances from the opposite window surfaces, the relative temperatures of these surfaces may be made to differ under given ambient conditions. I

These, and other features, objects and advantages of the invention, including certain details of the preferred form thereof, will become more fully evident by reference to the accompanying drawings.

Figure 1 is a front view of a window incorporating one type of electrode grid capable of producing the desired stray-field heating effect, with the electrodes connected to a high-frequency energy source.

Figure 2 is a fragmentary sectional view of a window incorporating an electrode grid comprising transparent electrically conductive strips of straight linear form arranged in parallel relationship, the view being taken transversely to the length of the electrode strips.

Figure 3 is a similar sectional view in which the electrode grid is formed of fine wire conductors.

Figure 4 is a front view of a window incorporating a modified type of electrode grid producing the desired stray-field heating effect, the electrodes being connected to a high-frequency energy source.

Referring to Figure 1, the generally rectangular window 10 mounted in a surrounding housing wall 12, such as an airplane cabin, incorporates a plurality of elongated conductive strips 14 arranged in parallel series relationship to form an electrode grid embedded in the window parallel to its major faces. A conductor 14a extends along one edge of the window, to which the electrode strips are generally perpendicular, and is connected electrically to alternate electrode strips commencing with that lowermost in the series. A similar conductor 14b extends along the opposite edge of the window and is connected electrically to the intermediately situated alternate electrode strips, that is, the second, fourth, etc., from the lowermost. An interdigitated electrode grid arrangement is thereby formed in a general plane generally parallel to the window surfaces. The conductors 14a and 14b are connected to 3 respectively opposite terminals of the high-frequency electrical energy source 16.

The high-frequency electrical energy source 16 may be of any suitable type capable of producing the necessary high-frequency alternating voltages for dielectric heating: purposes. The'heat generated in the window 10 by dielectricenergy l-oss'will' be proportional to the product of the energy source frequency and the square of the effective or average voltage gradient existing in the window material. 'For controlpu-rposes it is desirable that the energy source 16 be provided with an adjusting element 16a for varying 'the'source frequency, and a second adjusting element 16b for varying the'output voltage of the source. If desired, automatic regulating means may be incorporated in the-energy source responsive to a temperature sensitive element (not shown) embedded in or near the windowfor sensing variations of window temperature and thereby producing follow-up control adjustments of source frequency or voltage in order to maintain a substant-ial-ly constant anti-icing temperature in the window.

Thedetails of suitable energy source circuits and control provisions therein which ma be used in conjunction with y the illustrated dielectric heating anti-icing apparatus are 'not shown or described herein, since they are not neces sary-to an understanding of the present invention and suitable types are well known in the art.

ln'Figure 2 the elongated and relatively narrow electrode strips 14 are shown in cross section as being received between the mutually bonded panels lltla and lilb comprising the window it). In this figure the curved dotted lines which terminate upon the successively adjacent electrode strips are intended to represent the lines of dielectiic flux produced in the window as a result of high-frequency voltage appliedto such strips according to the arrangement described above. It will be seen that-these dielectric lines offorce extend not only in theregions dir'e ctly between the adjacent strips, but fringe or stray outwardly through the window material to the outside surfaces thereof, so that dielectric heating takes place in all regions of the window. It is true that the more intense heating will take place in the spaces lying directly between the strips and that the-amount of heat generated in a unit volume of window material decreases outwardly toward the window surfaces as a result 'of the decreasing voltage gradients; however, the existence of some temperature I differentials in the window is not a serious shortcoming if, asis readily possible, theEminimu-m window temperature at all areas of the exposed window surface'is adequate for anti-icing purposes. -While the areas directly outside the longitudinal central regions of-the conductors are not heated as greatly dielectrically, a certain amount of heat conduction to these areas will take place tending to equalizc temperature over --theentire window surface to be heated.

It will be noted'in Figure 2that, as 'previously-m'en- -tioned, dielectric force lines stray from the plane of the electrode grid to both faces of the window. By properly selecting the respective thicknesses of the panels 10b and 10a, it is readily possible to produce a greater dielectric heating efiect at 'one surface- (that nearest-the plane of the grid) than*at--the other. Thus, if the exposed surface of the panel ltlb'represents the exteriorsur- 'face 'ofthe' window upon which-ice might form-except for 'heatgenerated in the'window, the panel 1% may be made thin relative to the panel 10aso that'thegreatest amount of heating takes place atthe cxposed -surface of the former. 'Nevertheless, some heat will be generated at the exposedsurface of'panel itllaand in-the case of a vehicle, for instance, this-heat in--a properly designed installation would serve the usefulpurpose of "preventing the condensation of" moisture on'the window and thereby eliminatethenecessity for s'eparate de frosters, usually used 'in conjunction withvchicle"Windshields.

InQ-Figdre 3 an alter-native form of electrode grid' structure'is-"shown. 5 In this case, the elongatedconductive 7 strips forming the grid electrode conductors comprise the fine conductive wires 1411 between which the dielectric lines of force extend when voltage is applied to the grid as in the preceding case. An advantage of the form shown in Figure 3 is that the expensive methods of producing microscopically-thin transparent electrically conductive films are avoided and the window may be cast or molded in a single panel with the grid of conductors embedded in the panel at the desired location. The wires Me will not be noticeable for most purposes since they need carry very little electrical current, hence may be of a very fine gauge.

In Figure 4 there is illustrated a modified type of electrodegrid arrangement in a window It). In this case the grid is formed as a decreasing spiral vby two continuous conductors l8 and 20, connected to respectively opposite terminals of the high-frequency energy source 16, and extending in parallel relationship around the inside of the window. The spiral grid arrangement formed by these electrode conductors is such that a transverse section taken through the'window intermediate its opposite edges would reveal an arrangement of electrode .conductorswith alternately opposite plurality similar to that shown in Figure 2 or 3, depending upon the type of electrode conductors used in this instance. The heating effect would be substantially the same as in the precedingform since thespacing between conductors and the distances separating-the general plane of the electrode grid from the respectively opposite-window surfaces could be made the samc'for anygiven installation in either case.

In the event the particular form of electrode grid selected for present purposes utilizes grid conductors of the microscopictransparent conductor film material known in the art, it will be evident that the desired grid con figuration may be formed by either of different methods.

For instance, anentire window panelsurface may be coated with the-transparent film material and the grid configuration formed from this coatingby an etching process-removing only the film in selected areas and leaving the window-materialunaffected. Alternatively, the selected figuration of strip conductors may =beformed bya polishing process to remove the conductive material in selected areas. If desired the conductor 14a and the electrode strips connected thereto could be formed on one of the panels 10a or 19b, while the conductor 14b and connected electrode strips could be formed on the other panel. The interdigitated arrangement of electrode strips would then be achieved by bonding the two panels together in proper registry.

These, and other specific aspects and variations ofthe described invention will be evident to those skilled in the art.

I claim as my invention:

lQWindow anti-icing means comprising, in combination with a window of dielectric material, a plurality of plied with alternately opposite polarity to successively adjacent conductors in said grid, at source of high frequency electrical energy having output terminals, and means connecting said conductors to said source terminals with successively adjacent conductors in said grid having opposite polarity, the frequency and voltage of said energy 'source being sufiicient to heat the window dielectrically to-anti-icing temperature along the exposed surface area thereof.

2. The anti-icing means defined in claim 1, wherein U the window comprises two panels bonded together and the electrode grid conductors comprise transparent electrically conductive coating strips formed on the interface of one of said panels.

3. The anti-icing means defined in claim 1, wherein the window comprises a panel molded in a single thickness and the electrode grid conductors comprise fine wires embedded in said panel.

4. The anti-icing means defined in claim 1, wherein the conductors making up the electrode grid are arranged in interdigitated relationship, alternate conductors being connected together electrically at respectively opposite sides of the grid.

5. The anti-icing means defined in claim 1, wherein the conductors making up the electrode grid comprise a pair of conductors arranged in the form of a spiral with the conductor portions in successive turns of the spiral being spaced substantially uniformly from those in successively adjacent turns thereof.

6. Window anti-icing means comprising, in combina tion with a window of dielectric material, a grid of electrical conductors incorporated within said window, the grid conductors being spaced apart in substantially parallel relationship in a general plane substantially parallel to an exposed surface of said window, to define a multiplicity of interconductor spaces extending over a window area to be heated for anti-icing purposes and into which dielectric lines of force extend throughout the thickness of said window when voltage is applied with alternately opposite polarity to successively adjacent conductors in said grid, 21 source of high frequency electrical energy having output terminals, and means connecting said conductors to said source terminals with successively adjacent conductors in said grid having opposite polarity, the frequency and volt age of said energy source being suflicient to heat the window dielectrically to anti-icing temperature along the exposed surface area thereof.

7. The anti-icing means defined in claim 6, wherein the window comprises two panels bonded together and the electrode grid conductors comprise transparent electrically conductive coating strips formed on the interface of one of said panels.

References Cited in the file of this patent UNITED STATES PATENTS 2,228,136 Hart Jan. 7, 1941 2,412,982 Hart Dec. 24, 1946 2,449,317 Pitman Sept. 14, 1948 2,569,773 Orr Oct. 2, 1951 

